1. Field of the Invention
The present invention relates to a photoinduced current analyzer and, more particularly, to a scanning photoinduced current analyzer capable of detecting photoinduced current without supplying any steady-state current to a measuring object.
2. Description of the Related Art
Generally interconnection lines of a material containing aluminum (Al) as a principal component are used for its low specific resistance and its ease in processing for interconnecting the elements of a semiconductor integrated circuit, such as transistors and resistors. Deterioration of the reliability of Al interconnection lines has become more serious with the progressive miniaturization of semiconductor integrated circuits and increase in interconnection levels of semiconductor integrated circuits. Deterioration of the reliability of Al interconnection lines is attributable to increase in current stresses and mechanical stresses induced in Al interconnection lines.
While the size of Al interconnection lines is on the submicron order, currents that flow through Al interconnection lines are on the order of several hundred microamperes and current density is as high as the order of 105 A/cm.sup.2. Mechanical stresses on the order of 100 MPa are induced in interconnection lines when the semiconductor integrated circuit is subjected to heat treatment in the LSI manufacturing process.
Such stresses induced in the interconnection lines cause the migration of Al atoms, electromigration or stress migration, which forms voids in the interconnection lines, and voids increases the resistance of the interconnection lines and, in the worst case, breaks the interconnection lines.
Accordingly, the detection and observation of the density and positions of voids in the interconnection lines are essential to the enhancement of the reliability of Al interconnection lines. A surface potential contrast method is a known method of detecting voids in Al interconnection lines using a SEM (Scanning Electron Microscope) or a FIB apparatus (Focused Ion Beam apparatus). However, the surface potential contrast method must be carried out in a vacuum and is capable of analyzing only exposed interconnection lines and of detecting only completely broken parts in interconnection lines.
For example, an OBIRCH method (Optical Beam Induced Resistance Change Method) disclosed in Proceedings of the 19th International Symposium for Testing & Failure Analysis, pp. 303-310 (1993) is not subject to those restrictions.
FIG. 20 is a block diagram of an apparatus for carrying out the OBIRCH method.
First the operation of the apparatus will be described.
A current is supplied to a sample to be measured 6 from a dc power supply 8, and a current amplifier 10 monitors the current.
A laser scanning microscope focuses a laser beam on the sample to be measured 6, and scans an area specified on the sample to be measured 6 by a signal r provided by a controller 20 with the laser beam.
For example, when the laser scanning microscope is provided with an He--Ne laser that emits laser light of 633 nm in wavelength, the laser beam can be focused on the sample to be measured 6 in a spot of about 0.5 .mu.m in diameter.
An image information converter 12 receives a laser beam scanning position signal 1 from the controller 20 and a signal i corresponding to a current flowing through the sample to be measured 6. The image information converter 12 provides image information corresponding to the signal i at each scanning position, such as a signal b representing luminance corresponding to the current signal i, and then an image output unit 14 provides an image corresponding to the signal b.
FIG. 21 is a diagrammatic view of the sample to be measured 6. Since the OBIRCH method needs to supply a current to the sample to be measured 6 as mentioned above, the measuring interconnection lines must be formed in a measuring pattern comprising a interconnection pattern 16 and terminal pads 18.
FIG. 22 is a block diagram of the laser scanning microscope 2. A laser beam 4 emitted by a laser light source 100 falls on an optical deflector 102. The optical deflector 102 deflects the laser beam 4 by an angle specified by the signal r provided by the controller 20. A focusing unit 106 focuses the laser beam 4 on the sample to be measured 6.
Since the position of the spot of the focused laser beam 4 on the sample to be measured 6 is dependent on the deflection angle, the laser beam 4 is deflected according to the signal r for scanning. In the laser scanning microscope 2 shown in FIG. 22, the focusing unit 106 comprises a reflecting mirror 58 and an objective lens 56.
An image forming unit 104 is disposed on an optical path along which the laser beam 4 travels to form an image by focusing the reflected light reflected by the sample to be measured 6 and to provide an image signal v. In this example, a beam splitter 50 reflects part of the reflected light and an image forming lens 52 forms an image on a photodetector 54. The photodetector 54 may be, for example, a linear CCD (Charge-Coupled Device). The photodetector 54 provides the image signal v.
The image information converter 12 receives the image signal v. The image information converter 12 provides either the image information signal b or the image signal v according to the control signal 1 provided by the controller 20.
The void detecting principle of this apparatus for detecting a void formed in an interconnection line 16, such as an Al interconnection line, will be described hereinafter with reference to FIG. 23.
Suppose that, in a reference state, the laser beam is projected on a region P in which no void is found in the Al interconnection line. When the laser beam falls in a region Q in which voids are formed, heat is transferred at a reduced rate. Therefore, a temperature rise .DELTA.T.sub.Q in the region Q when irradiated with the laser beam is greater than a temperature rise .DELTA.T.sub.P in the region P when irradiated with the laser beam and, consequently, the range of variation of resistance is wider when the region Q is irradiated with the laser beam than that when the region P is irradiated with the laser beam. Therefore, the regions P and Q differ from each other in current change .DELTA.I detected by monitoring the current I flowing through the interconnection line, an output image which enables the determination of the position of a void can be obtained by giving image information obtained by converting the current change .DELTA.I at a scanning position into corresponding luminance data to the image output unit 14, such as a CRT.
The position of the void in the Al interconnection line 16 can be determined through the comparison of the output image with an image output based on the image formation data provided by the laser scanning microscope 2.
Since this method determines the change (increase) in the resistance of the region having the void due to heat generation from change (reduction) in the current flowing through the sample to be measured 6 and forms an image, a voltage must be applied to the interconnection line to supply a current on the order of several milliamperes for analysis. Accordingly, sample to be measured having a high resistance of about 10 k.OMEGA. or above cannot be analyzed and the method is subject to restriction on the resistance of the sample to be measured. Since it is difficult to apply a voltage to a desired interconnection line of a device, such as an LSI circuit, to supply a current to the desired interconnection line, it is scarcely possible to apply this method to the analysis of practical devices.